1. Field of the Invention
The present invention relates to a camera data communication system for performing data communication operations between a detachable external accessory and a camera body through a data communication line (or lines) and for performing data communication operations between a camera data communication device and a circuit unit in the camera body through the data communication line (or lines).
2. Related Background Art
Various kinds of accessories attached to a camera, such as an interchangeable lens, a speedlight and a data back, have been manufactured, and there has been a recent tendency to provide these accessories with many functions in order to achieve more advanced control operations when the accessories are combined with the body of the camera. For that purpose, it is necessary to electrically connect the camera body and the accessories to each other and to transmit and receive various kinds of data by serial data communication.
Furthermore, there is a general tendency for a CPU serving as a nucleus inside the camera body to exchange data with internal units, such as an EEPROM for performing nonvolatile storage of set information and a driver IC for driving a display, by serial communication. Therefore, it is required that the CPU in the camera body be able to execute serial data communication with a plurality of other internal units and with accessories in order to transmit and receive information necessary for control operations.
Although it is preferable in the above serial data communication that the CPU itself have many communication lines, the number of the lines is generally limited. Therefore, basic lines, such as synchronizing clocks (in the case of synchronous communication), a data input and a data output, are connected in a row to all of the other internal units and to the accessories. A signal for selecting one of these internal units and one of the accessories is generated for each of the accessories or the internal units, and thereby only the selected unit or accessory can use the communication lines. The CPU in the camera body thus communicates with all the other internal units and with the accessories during time sharing.
However, since the accessories are detachably mounted on the outside of the camera, contacts for electrically connecting the accessories and the camera to each other are likely to be exposed when the accessories are not attached to the camera. For example, a hot shoe contact for attaching a speedlight to the camera is always exposed, and a contact disposed in a mount for an interchangeable lens is also exposed when the lens is not attached to the camera during an interchange operation.
If an exposed contact is touched by a hand or splashed with rainwater, if contacts of the camera body and the accessory which do not correspond to each other are brought into contact with each other when the accessory is interchanged with another accessory, or if the contact is short-circuited to the camera housing, any of the above communication lines is short-circuited to the camera housing. This leads to incorrect recognition of the internal unit or the accessory corresponding to the short-circuited contact, and furthermore, hinders communication with the other internal units and the accessories because all the communication lines are connected in a row as described above.
As a result, the following two problems are likely to arise: first, communication with any of the accessories or internal units is made impossible, for example, when synchronizing clocks are short-circuited, and second, communication is formally valid, but transmitted and received data is not reliable at all, for example, when the data communication lines are short-circuited.
As regards the first problem, since communication is impossible and the camera does not operate at all, the user can notice that a malfunction of the camera has occurred and take some corrective measures. However, as regards the second problem, the camera outwardly looks normal, but transmitted and received data is not reliable at all. Since the camera or the accessory continues to operate based on incorrect information, the user will continue taking pictures without being aware of the malfunction until after the pictures are developed. This second problem is thus worse than the first one.